My friends and I were playing Obstacle Course on split-screen in version 1.3 and a glitch happened. It was on Pompeii, but I suspect it's universal. This is what I think happened, step by step:

1. Player 2 fell off the map into the lava and their boulder was destroyed2. Player 1 crossed the finish line3. Player 2 respawned, even though the round had already ended4. A new round began5. Player 2's screen was now completely black, though they could still control their boulder and be seen normally on player 1's camera

I'm not entirely sure of the order of the first 2 steps, but I'm sure player 2 respawned after the round was over because their boulder was destroyed right around the time player 1 crossed the finish line.